The Nepal Flood Reveals Growing Risks To Hydropower From Glacial Retreat

A landslide and glacier collapse caused a flash flood in Nepal, killing at least 1,400 people and damaging 13 hydropower projects (Photo by Prabin Ranabhat / AFP via Getty Images)

AFP via Getty Images

Glaciers and ice are retreating in high mountain regions around the world, melting and shrinking in the face of a warming climate.

This retreat is occurring at timescales we experience as relatively slow—decades to centuries—and that loss of ice will be felt in the form of drought, a steady decline in the river baseflows needed by people for drinking, irrigation and hydropower.

But sometimes that retreat happens fast, with abrupt collapses or sudden ruptures. Those are experienced as floods and unfold as deadly disasters for people and catastrophic losses for infrastructure.

In late August, a deadly and catastrophic collapse of a mountainside and glacier in the Himalayas unleashed disaster along the Bhotekoshi and Trishuli rivers in Nepal, with thousands of people dead or missing and billions of dollars in damages to roads, bridges, buildings and hydropower dams.

In the short term, Nepal will focus on supporting the victims and rebuilding the region. Meanwhile, as the world mourns the loss of life in Nepal, it should also grapple with the fact that this event is yet one more example of how climate change is increasing risks and harms to low- and lower-middle-income countries that have barely contributed to the global buildup of greenhouse gases.

In the long term, the melting of the high-altitude cryosphere (glaciers, ice, snow and permafrost) will have major impacts on communities and infrastructure within alpine valleys and in downstream regions that depend on the rivers that flow out of mountains. For example, the planning, construction, and operation of hydropower dams will need to fully account for this shrinking – the steady melting, year after year toward a drier future, yet punctuated by sudden surges of floodwaters. These new realities should shift how energy systems are planned and built.

On August 26, seismometers around the world detected a major event from the Tibet-Nepal border region. It was initially classified as an earthquake, but scientists soon realized the tremors were actually from a landslide, one massive enough to generate an earthquake-like signal (5.2 on the Richter scale). Indeed, the landslide was one of the largest the world has seen over the past few decades, a wall of rock and glacier that rushed more than a kilometer down the mountainside. Traveling at 150 kilometers per hour, the landslide crashed into the valley floor, releasing more energy than the atomic bomb used in Hiroshima.

The kinetic energy of the slide melted much of the glacial ice and, upon landing in the canyon bottom, the rock debris dammed the Purepu Tsangpo River in Tibet. The dam soon was breached, unleashing a slurry of rock, ice, water and mud hurtling downstream toward the border crossing with Nepal, and then toward Nepalese towns along the Trishuli River. Videos of this black wall of muddy water and debris—initially 70 m (230 feet) high and moving 180 kilometers per hour (110 mph)— are truly horrifying as it engulfs towns, dislodges bridges and picks up and sweeps away buses and buildings.

As of this writing, nearly 1,400 people are dead from the flood with approximately 5,000 still missing. Total damages are estimated at approximately $5 billion, which is about 10% of Nepal’s Gross Domestic Product (in April 2015, an earthquake struck Nepal that caused $5 to $10 billion in damages).

The damage included at least 13 hydropower projects, with some totally destroyed. Nepal’s National Energy Agency estimates the damage to the hydropower sector at “hundreds of billions of rupees” – into the billions of dollars.

Altogether, at least 430 MW of hydropower capacity was destroyed or damaged, more than 10% of Nepal’s total hydropower capacity. And because Nepal’s grid capacity is almost entirely from hydropower, that means more than 10% of the total grid capacity was destroyed or damaged.

Along with the existing capacity, five hydropower projects under construction, representing hundreds of additional MW, were also damaged or destroyed, and hundreds of workers from these projects are among the dead and missing.

For Nepal, hydropower has been its energy past, present and future. That technology provides 97% of its current electricity capacity and, as Nepal plans on a large expansion of its grid capacity (by three to seven times), in part to allow major exports to India and other neighbors, nearly all of that expansion is planned to come from hydropower.

Nepal focuses on hydropower because it has its two primary resources in abundance: water and steep topography producing large elevation drops.

But the recent flood disaster illustrates that those two resources are also increasingly becoming risks. The steep topography is crumbling more frequently, turning water into muddy walls of destruction.

In the past few years, Nepal has experienced several flash flood events caused by melting ice and/or landslides, as have other parts of the Himalayas (note that melting permafrost—which acts as a glue to hold steep slopes together—can increase the risk of landslides). In 2021, the Indian Himalayas experienced a disaster very similar to last month’s flood in Nepal. In Uttarakhand state, a sudden landslide of rock and glacier caused a flood and debris flow that killed more than 200 people – most of them workers at two hydropower dams that were severely damaged.

Another type of risk is that from Glacial Lake Outburst Floods (GLOF), the sudden draining of a high-elevation lake that is dammed behind a glacier, with the breaching sometimes triggered by a landslide or seismic event. Some of the largest peak river flows on Earth have been caused by GLOFs.

In 2023, a GLOF in the Indian Himalayas traveled nearly 400 km downstream, killing 55 people and obliterating the Teesta-3 dam, a 1200 megawatt hydropower dam that was only a decade old and cost 1.5 billion USD to construct.

In addition to the danger to people, the risk of GLOFs and other disasters driven by glacial retreat poses a serious challenge for high mountain countries around the world that depend on hydropower – currently and/or within expansion plans.

While the Himalayas have the greatest number of people at risk from GLOFs, Peru is the country with the third highest number of people at risk from these events, suggesting that Peru’s hydropower projects are also at risk. And although Peru’s power system is far more diversified than Nepal’s, it also has a high number of planned hydropower dams.

In a 2022 analysis I led on flood risks for hydropower, we found that Nepal and Peru are among the countries with the highest current flood risk, with both categorized as “high risk,” whereas the global average for hydropower projects falls within “medium risk.” Those two countries are also projected to have among the highest increases in risk due to climate change (see below).

But fast-moving floods are just part of the risk picture. There is also the steady decline of high-elevation snow, ice and glaciers. Portions of high-elevation ice and snow have always melted each summer and that meltwater often provides a primary source of water for snow-fed rivers during periods of the year with limited precipitation. But as the total amount of high-elevation ice diminishes, there will be less meltwater available to feed those rivers during the dry season.

Studies for Nepal suggest that total hydropower generation potential could decline by 30% by the end of the century (though this decline will be preceded by a temporary period of increased generation during the accelerated melting). The Andes are already experiencing droughts that have dramatically reduced hydropower generation, with Ecuador enduring power cutoffs of 14 hours every day in the fall of 2024.

Hydropower in mountainous regions—both current and planned expansions—will clearly need to adapt to this glacial shrinking and associated risks of both floods and declining baseflows.

Countries should fully consider these risks when planning climate-resilient power systems. Both research and real-world examples demonstrate the benefits of diversified power systems for managing water scarcity.

For example, the World Bank noted that, in Kenya, “development of geothermal, wind, and solar energy generation has enhanced energy security and significantly reduced the kind of weather-induced supply shortages typical in systems with a large share of hydropower.”

A research paper in Nature Water examined the recent drought in Ecuador and concluded that “a strong build-out of solar and wind power…could become an important element in the fortification of Ecuador’s power system against similar future droughts.”

Strategic energy planning can be used to identify the mix of technologies and projects needed to build a climate-resilient grid. Note that this approach can also be key for minimizing negative impacts from poorly sited hydropower dams on river ecosystems and the people that depend on them.

Both Nepal and Peru will likely benefit from diversifying future energy expansion plans to include more wind, solar and batteries. Hydropower dams that are built should be carefully screened so that projects are not developed in areas with high risk of GLOFs or other hazards.

One specific type of hydropower project may become increasingly important to climate-resilient grids: pumped storage hydropower (PSH). With PSH, water is pumped uphill during periods when energy is plentiful and stored in an upper reservoir. When energy is needed, the water can flow back downhill to generate electricity for the grid. This storage and energy-on-demand service can be particularly valuable in a diversified grid with variable sources such as wind and solar.

Because one, or even both, of the reservoirs of PSH can be built away from river channels, PSH has considerably lower risks from floods than does conventional hydropower. And because PSH involves recirculating a volume of water, rather than depending on incoming baseflows, it can also be far more resilient to periods of water scarcity.

As glaciers and ice retreat, countries with high-elevation mountain areas will need to find ways to minimize risks to people – and, under the principle of climate justice, other countries and financial institutions should support Nepal and other vulnerable countries in this effort.

Power systems also confront a range of risks from glacial retreat, but a combination of system diversification, strategic planning, careful siting, and other best practices can help design and build grids that are far more resilient to those risks.